This very much captures what I am working on. It is (currently java) a sdk dependency + ide plugin, which records input/return values of Java methods. Once recorded you can either instantly replay it on a running process (also works on hot-reloads) OR save it as a fixture which you can replay after a process restart. In our workflow we save these fixtures to git so anyone in the team can replay them. But there are a couple of things I feel are needed on top of this

  - a flexible assertion framework, currently I am doing a straightforward string equals to identify breaking changes
  - a mocking feature for downstream calls, currently nothing is mocked out so it is basically an integration test, but as the discussion shows, mocking can be quite handy in a variety of situations